The present invention relates to micro circuits and pertains particularly to the packaging and fabrication of and mounting of certain dies or chips.
Many electronic devices such as power transistors require a heat sink in order to dissipate or transfer heat from the system during operation. The chip or die is bonded to a heat sink for this purpose. The bonding of the transistor or die element of the transistor to a heat sink is critical in order to assure adequate heat transfer from the transistor or die.
In the production of power transistors, a die is mounted by soldering directly to a heat sink which may also form a portion of the lead frame for the package assembly. The soldering of the die to the heat sink as pointed out above is critical in order to assure a uniform transfer of heat from the die to the heat sink.
In the conventional technique of soldering such dies or chips to the heat sink, the heat sink is loaded on a conveyor to be passed through a furnace during which time the solder is melted and the die is bonded to the heat sink. This operation in the conventional or prior art approach involves the loading of heat sinks onto a conveyor and the hand loading of solder pre-forms on the respective heat sinks and thereafter the loading of a die or chip onto the pre-form with a weight loaded onto the chip. As illustrated in FIG. 1, for example, a heat sink or lead frame 10 is loaded onto a conveyor to pass through a furnace. A solder pre-form 12 is placed on the heat sink in the position where the die is to be bonded. Thereafter a die or chip 14 is then placed on top of the solder pre-form 12 and a weight 16 placed on top of the die. This is all done manually with the orientation and placement of the die 14 being critical. In the conventional approach this hand loaded assembly is then passed through a furnace which heats the assembly to a temperature that melts the solder 12, allowing the die 14 then to be pressed down by the weight 16 into contact with the lead frame or heat sink 10. Frequently this approach results in an inefficient bonding, such as illustrated in FIG. 2, where the die 14 rides up on top of the solder 12 at an angle, which is exaggerated for illustrative purposes. This results in a poor bonding of the die to the heat sink. Also gas bubbles may form between the die, and the solder or the lead frame, resulting in a poor heat transfer path.
The prior art approach resulted in higher cost production because of the low unit production by the hand assembly method and a high expense, not only because of the labor but also because of the excessive amount of forming gas required for the typical furnace. Furnaces for such assembly are typically on the order of approximately 30 feet in length and require 150 cubic feet per hour of nitrogen on each end of the furnace and a 150 cubic feet per hour of hydrogen inside of the furnace. The unit throughput per hour of such systems was on the order of approximately 350 units per hour.
In addition to the low production rate and high cost as a result of the high gas usage, the weights on top of the dies would frequently move causing a misorientation as well as uneven soldering of the die. This resulted in rejects of a fairly high percentage of the production due to the poor bonding as well as the possible misorientation of the die. Frequently a die assembly will emerge from the furnace with the die 14 misoriented as illustrated in FIG. 3. Wire bonding of the lead frame to the die becomes difficult under such circumstances and frequently cannot be accomplished with automatic bonding equipment.
The present invention was conceived and developed to overcome the above and other problems of the prior art.